VehicleMonitor-WEB/node_modules/deasync

DeAsync.js

DeAsync turns async function into sync, implemented with a blocking mechanism by calling Node.js event loop at JavaScript layer. The core of deasync is written in C++.

Motivation

Suppose you maintain a library that exposes a function getData. Your users call it to get actual data:
var myData = getData();
Under the hood data is saved in a file so you implemented getData using Node.js built-in fs.readFileSync. It's obvious both getData and fs.readFileSync are sync functions. One day you were told to switch the underlying data source to a repo such as MongoDB which can only be accessed asynchronously. You were also told for backward compatibility, getData API cannot be changed to return merely a promise or demand a callback parameter. How do you meet both requirements?

You may tempted to use node-fibers or a module derived from it, but node fibers can only wrap async function call into a sync function inside a fiber. In the case above you cannot assume all callers are inside fibers. On the other hand, if you start a fiber in getData then getData itself will still return immediately without waiting for the async call result. For similar reason ES6 generators introduced in Node v0.11 won't work either.

What really needed is a way to block subsequent JavaScript from running without blocking entire thread by yielding to allow other events in the event loop to be handled. Ideally the blockage is removed as soon as the result of async function is available. A less ideal but often acceptable alternative is a sleep function which you can use to implement the blockage like while(!done) sleep(100);. It is less ideal because sleep duration has to be guessed. It is important the sleep function not only shouldn't block entire thread, but also shouldn't incur busy wait that pegs the CPU to 100%.

DeAsync supports both alternatives.

Usages

• Generic wrapper of async function with conventional API signature function(p1,...pn,function cb(error,result){}). Returns result and throws error as exception if not null:

var deasync = require('deasync');
var cp = require('child_process');
var exec = deasync(cp.exec);
// output result of ls -la
try{
    console.log(exec('ls -la'));
}
catch(err){
    console.log(err);
}
// done is printed last, as supposed, with cp.exec wrapped in deasync; first without.
console.log('done');

• For async function with unconventional API, for instance function asyncFunction(p1,function cb(res){}), use loopWhile(predicateFunc) where predicateFunc is a function that returns boolean loop condition

var done = false;
var data;
asyncFunction(p1,function cb(res){
    data = res;
    done = true;
});
require('deasync').loopWhile(function(){return !done;});
// data is now populated

• Sleep (a wrapper of setTimeout)

function SyncFunction(){
  var ret;
  setTimeout(function(){
      ret = "hello";
  },3000);
  while(ret === undefined) {
    require('deasync').sleep(100);
  }
  // returns hello with sleep; undefined without
  return ret;    
}

Installation

Except on a few platforms + Node version combinations where binary distribution is included, DeAsync uses node-gyp to compile C++ source code so you may need the compilers listed in node-gyp. You may also need to update npm's bundled node-gyp.

To install, run

npm install deasync

Recommendation

Unlike other (a)sync js packages that mostly have only syntactic impact, DeAsync also changes code execution sequence. As such, it is intended to solve niche cases like the above one. If all you are facing is syntactic problem such as callback hell, using a less drastic package implemented in pure js is recommended.

Support

Pull requests and issue reporting are welcome. For issues to be considered by maintainer

1. they must be reproducible
2. there must be evidence the issue is related to DeAsync

To that end, the issue should contain platform information, error message relevant to DeAsync, and preferably code snippet. If code snippet is supplied, it must be self-contained, i.e. independent from your runtime environment or other modules not explicitly specified via require in the code snippet.

License

The MIT License (MIT)

Copyright (c) 2015

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.